This application claims Paris Convention priority of DE 199 04 880.0 filed Feb. 6, 1999 the complete disclosure of which is hereby incorporated by reference.
The invention concerns an opto-electronic sensor, preferentially for measuring the opaqueness of a medium, having a transmitter channel for transmitting light waves along a first axis and at least one receiver channel for detecting light waves which have interacted with the medium, wherein the receiver channel cooperates with an evaluation unit. The invention also concerns a method for operating an opto-electronic sensor.
Sensors of this type are used to measure the properties of fluids, in particular, in the food and chemical industry and are often used for continuous measurements in processing technology. The above mentioned applications have stringent requirements for the temperature stability and the resistive strength to pressure as well as the stability with regard to aggressive chemicals. These requirements result, i.e. from the need to clean and sterilize sensors used in these food and chemical industries.
Opto-electronic sensors have the inherent problem of requiring optical isolation between the transmitter and receiver channels. Transmitter channel light waves are modulated for identification purposes to facilitate detection by the evaluation unit. Under all circumstances, light waves must be prevented from leaving the transmitter channel and gaining direct access to the receiver channel without having interacted with the medium, since this would falsify or prohibit measurement thereof.
This can be achieved when the transmitter channel and the receiver channel each have their own housing. However, this complicates the sensor and results in increased difficulty and expense. Alternatively, one can dispose both the transmitter channel and the receiver channel within one housing and separate them from each other using optically opaque materials. Although this is relatively easy to do inside of the sensor housing, there are problems at the interface or exit location into the medium to be measured. Two separate windows made from one optically transparent material are required which increases the difficulty and expense of the seal.
Departing therefrom it is the underlying purpose of the present invention to improve an opto-electronic sensor of the above mentioned kind as well as a method for operation of a sensor of this type in such a fashion that the sensor is more compact and can be manufactured more economically to reduce the degree of difficulty and expense of optical isolation between the transmitter channel and the receiver channel and for sealing the sensor housing with respect to the medium to be measured.
This purpose is achieved in accordance with the invention with a sensor of the above mentioned kind in that a common window is used for sealing the transmitter channel and the receiver channel with respect to the medium to be measured as well as for coupling in and out the light waves, the window being made from an optically transparent material, wherein the inner border surface of the window facing the receiver channel is adjacent to an optically thinner medium having a lower optical index of refraction than that of the window material and is oriented in such a manner with respect to the outer border surface of the window adjacent to the medium to be measured as well as with respect to the orientation of the first axis relative to this outer border surface, that light waves from the transmitter channel which are reflected back from the outer border surface into the window are incident on the inner border surface at an angle which is larger than the critical angle for total internal reflection so that the light does not enter into the receiver channel, but is reflected again. In accordance with the invention, the incident angle for the transmitter light beam is chosen in such a fashion that light from the transmitter reflected from the outer border surface into the window is incident on the inner border surface at an angle which exceeds the total internal reflection angle.
The light is reflected and thereby does not enter into the receiver channel.
The additional purpose is solved with a method having the features of the independent method claim.
The solution in accordance with the invention facilitates use of one and the same exit location (the window) both for the transmitter channel and for one or a plurality of receiver channels without having to provide for optically opaque materials in the form of walls or the like between the receiver and transmitter channels. Irradiated light waves from the transmitter channel which did not enter into the measuring media but were reflected back at the border surface with the medium cannot enter into the receiver channel. These light waves are totally internally reflected at the inner surface of the window facing the inside of the sensor, proximate the transmitter channel. This is facilitated by providing for an optically thinner material (preferentially air or another gas) at this inner border surface, i.e. inside the sensor. Light waves incident on this inner border surface at an angle of incidence in excess of the critical angle for total internal reflection are once more reflected into the window towards the outer border surface. It has turned out to be particularly advantageous when the window comprises a plane-parallel plate since, in this case, the back reflected portion is incident on the outer border surface at the same angle as the primary transmitter beam so that most of it exits. The portion which is once more reflected towards the inner border surface does however pass into the sensor, since the light waves are once more incident on the lower border surface at an angle which exceeds the critical angle for total internal reflection.
Light waves which, after interacting with the medium to be measured (e.g. through scattering with non-dissolved particles in a liquid), are incident from the outside onto the outer border surface of the window and enter into the optically transparent window material, then pass through the inner border surface into the sensor when they are incident at an angle which is less than the critical angle for total internal reflection. One orients the transmitter channel and thereby the first axis along which the light waves propagate within the window in such a fashion that the light is preferentially incident on the outer border surface at an angle between 45xc2x0 and 55xc2x0 relative to the normal. In this case, the so-called 90xc2x0 scattered light is incident on the inner border surface at an angle which is less than the critical angle for total internal reflection (assuming a watery measuring medium, an optically transparent window material made from glass as well as an optically thinner medium in the form of air inside the window).
It has also turned out to be advantageous when the first axis is oriented relative to the outer border surface in such a fashion that the angle xcex12, at which those light waves reflected back from the outer border surface into the window are incident upon the inner border surface, is larger than 42xc2x0. It has furthermore turned out to be advantageous when the angle of incidence xcex13, at which light waves which pass into the window due to interaction with the medium to be measured are incident on the inner border surface, is less than 40xc2x0, preferentially less than 34xc2x0.
Further features, details and advantages of the invention can be extracted from the accompanying claims and from the drawings as well as from the following description of a preferred embodiment of the sensor in accordance with the invention.